1. Field of Invention
This invention relates to novel, heat-developable photothermographic elements and in particular, it relates to phthalimide blocking groups for post-processing stabilizers for photothermographic elements.
2. Background to the Art
Silver halide-containing, photothermographic imaging materials (i.e., heat-developable photographic elements) processed with heat, and without liquid development, have been known in the art for many years. These materials, also known as "dry silver" compositions or emulsions, generally comprise a support having coated thereon: (a) a photosensitive material that generates silver atoms when irradiated; (b) a non-photosensitive, reducible silver source; (c) a reducing agent for the non-photosensitive, reducible silver source; and (d) a binder. The photosensitive material is generally photographic silver halide that must be in catalytic proximity to the non-photosensitive, reducible silver source. Catalytic proximity requires an intimate physical association of these two materials so that when silver atoms (also known as silver specks, clusters, or nuclei) are generated by irradiation or light exposure of the photographic silver halide, those nuclei are able to catalyze the reduction of the reducible silver source. It has long been understood that silver atoms (Ag.degree.) are a catalyst for the reduction of silver ions, and that the photosensitive silver halide can be placed into catalytic proximity with the non-photosensitive, reducible silver source in a number of different fashions. For example, catalytic proximity can be accomplished by partial metathesis of the reducible silver source with a halogen-containing source (see, for example, U.S. Pat. No. 3,457,075); by coprecipitation of silver halide and the reducible silver source material (see, for example, U.S. Pat. No. 3,839,049); and other methods that intimately associate the photosensitive, photographic silver halide and the non-photosensitive, reducible silver source.
The non-photosensitive, reducible silver source is a material that contains silver ions. Typically, the preferred non-photosensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of similar molecular weight are generally used. Salts of other organic acids or other organic materials, such as silver imidazolates, have been proposed. U.S. Pat. No. 4,260,677 discloses the use of complexes of inorganic or organic silver salts as non-photosensitive, reducible silver sources.
In both photographic and photothermographic emulsions, exposure of the photographic silver halide to light produces small clusters of silver atoms (Ag.degree.). The imagewise distribution of these clusters is known in the art as a latent image. This latent image is generally not visible by ordinary means. Thus, the photosensitive emulsion must be further processed in order to produce a visible image. The visible image is produced by the reduction of silver ions which are in catalytic proximity to silver halide grains bearing the clusters of silver atoms, i.e., the latent image. This produces a black-and-white image.
As the visible image is produced entirely by elemental silver (Ag.degree.), one cannot readily decrease the amount of silver in the emulsion without reducing the maximum image density. However, reduction of the amount of silver is often desirable in order to reduce the cost of raw materials used in the emulsion and/or to enhance performance. For example, toning agents may be incorporated to improve the color of the silver image of the photothermographic element. Another method of increasing the maximum image density in photographic and photothermographic emulsions without increasing the amount of silver in the emulsion layer is by incorporating dye-forming materials in the emulsion. Upon imaging, the leuco dye is oxidized, and a dye and a reduced silver image are simultaneously formed in the exposed region. In this way, a dye-enhanced silver image can be produced.
A number of methods have been proposed for obtaining color images with dry silver systems. Such methods include, for example, incorporating dye-forming coupler materials into the dry silver systems and color-forming dry silver systems include: a combination of silver benzotriazole, a magenta, yellow, or cyan dye-forming coupler, an aminophenol developing agent, a base release agent such as guanidinium trichloroacetate, and silver bromide in poly(vinyl butyral); and a combination of silver bromoiodide, sulfonamidophenol reducing agent, silver behenate, poly(vinyl butyral), an amine such as n-octadecylamine, and 2-equivalent or 4-equivalent yellow, magenta or cyan dye-forming couplers.
Color images can also be formed by incorporation of dye forming or dye releasing compounds into the emulsion. Upon imaging, the dye forming or dye releasing material is oxidized and a dye and a reduced silver image are simultaneously formed in the exposed region.
For example, leuco dye compounds are often incorporated into the emulsion. A leuco dye is the reduced form of a color-bearing dye. It is generally colorless or very lightly colored. Upon imaging, the leuco dye is oxidized and a dye and a reduced silver image are simultaneously formed in the exposed region.
Multicolor photothermographic imaging elements typically comprise two or more monocolor-forming emulsion layers (often each emulsion layer comprises a set of bilayers containing the color-forming reactants) maintained distinct from each other by barrier layers. The barrier layer overlaying one photosensitive, photothermographic emulsion layer typically is insoluble in the solvent of the next photosensitive, photothermographic emulsion layer. Photothermographic elements having at least two or three distinct color-forming emulsion layers are disclosed in U.S. Pat. Nos. 4,021,240 and 4,460,681. Various methods to produce dye images and multicolor images with leuco dyes are well known in the art as represented by U.S. Pat. Nos. 3,180,731; 3,531,286; 3,761,270; 4,022,617; 4,460,681; 4,883,747; and Research Disclosure March 1989, item 29963.
One common problem that exists with photothermographic systems is post-processing instability of the image and/or of the background following processing. The photoactive silver halide still present in the developed image may continue to catalyze formation of metallic silver during room light handling. This is known as "silver print-out." Thus, there exists a need for stabilization of the unreacted silver halide. The addition of separate post-processing image stabilizers or stabilizer precursors provides the desired post-processing stability. Most often these are sulfur-containing compounds such as mercaptans, thiones, and thioethers as described in Research Disclosure, June 1978, item 17029. U.S. Pat. No. 4,245,033 describes sulfur compounds of the mercapto-type that are development restrainers of a photothermographic system. See also U.S. Pat. Nos. 4,837,141 and 4,451,561. Mesoionic 1,2,4-triazolium-3-thiolates as fixing agents and silver halide stabilizers are described in U.S. Pat. No. 4,378,424. Substituted 5-mercapto-1,2,4-triazoles, such as 3-amino-5-benzothio-l,2,4-triazole, used as post-processing stabilizers are described in U.S. Pat. Nos. 4,128,557; 4,137,079; 4,138,265; and Research Disclosure 16977 and 16979.
In color photothermographic elements, often unreacted dye forming or dye releasing compounds may slowly oxidize and form areas of color in the unexposed areas. In these elements, stabilizers are often added to reduce "leuco dye backgrounding."
Some of the problems with these stabilizers include thermal fogging during processing or losses in photographic sensitivity, maximum density, or contrast at effective stabilizer concentrations.
Stabilizer precursors have blocking or modifying groups that are usually cleaved during processing with heat and/or alkali. This provides the primary active stabilizer that can combine with the photoactive silver halide in the unexposed areas of the photographic material to form a light- and heat-stable complex. For example, in the presence of a stabilizer precursor in which the blocking group on the sulfur atom is cleaved upon processing, the resulting silver mercaptide will be more stable than the silver halide to light, atmospheric, and ambient conditions.
Various blocking techniques have been used in protecting stabilizer precursors in photographic elements. Removal of these blocking groups from the photographically useful stabilizers is accomplished by an increase of pH during alkaline processing conditions of the exposed imaging material. Thus, U.S. Pat. No. 3,615,617 describes acyl blocked photographically useful stabilizers. U.S. Pat. Nos. 3,674,478 and 3,993,661 describe hydroxyarylmethyl blocking groups. Benzylthio releasing groups are described in U.S. Pat. No. 3,698,898. Thiocarbonate blocking groups are described in U.S. Pat. No. 3,791,830, and thioether blocking groups in U.S. Pat. Nos. 4,335,200, 4,416,977, and 4,420,554. Photographically useful stabilizers that are blocked as urea or thiourea compounds are described in U.S. Pat. No. 4,310,612. Imidomethyl blocked stabilizers are described in U.S. Pat. No. 4,350,752, and imide or thioimide blocked stabilizers are described in U.S. Pat. No. 4,888,268.
Blocking groups that are thermally-sensitive have also been used. These blocking groups are removed by heating the imaging material during processing. Photographically useful stabilizers blocked with thermally-sensitive carbamate derivatives are described in U.S. Pat. Nos. 3,844,797 and 4,144,072. These carbamate derivatives presumably regenerate the photographic stabilizer through loss of an isocyanate. Hydroxymethyl blocked photographic reagents that are unblocked through loss of formaldehyde during heating are described in U.S. Pat. No. 4,510,236. Substituted benzylthio releasing groups are described in U.S. Pat. No. 4,678,735; and U.S. Pat. Nos. 4,351,896 and 4,404,390 use carboxybenzylthio blocking groups for mesoionic 1,2,4-triazolium-3-thiolates stabilizers. Photographic stabilizers that are blocked by a Michael-type addition to the carbon-carbon double bond of either acrylonitrile or alkyl acrylates are described in U.S. Pat. Nos. 4,009,029 and 4,511,644, respectively. Heating of these blocked derivatives causes unblocking by a retro-Michael reaction. U.S. Pat. No. 5,158,866 describes the use of omega-substituted 2-propionamidoacetal or 3-propionamidopropionyl stabilizer precursors as post-processing stabilizers in photothermographic elements. U.S. Pat. No. 5,175,081 describes the use of certain azlactones as stabilizers. U.S. Pat. No. 5,298,390 describes the use of certain alkyl sulfones as blocked compounds capable of releasing stabilizers with heat. U.S. Pat. No. 5,300,420 describes the use of certain nitriles as blocked compounds capable of releasing stabilizers with heat.
Various disadvantages attend these different blocking techniques. Highly basic solutions that are necessary to cause unblocking of the alkali-sensitive blocked derivatives are corrosive and irritating to the skin. With the photographic stabilizers that are blocked with a heat-removable group, it is often found that the liberated reagent of by-product, for example, acrylonitrile, can react with other components of the imaging construction and cause adverse effects. Also, inadequate or premature release of the stabilizing moiety within the desired time during processing may occur, resulting in fogging of the emulsion or loss of sensitivity.
Blocking groups which are removed by actinic radiation are discussed in the context of organic synthesis by Amit et al. Israel J. Chem. 1974, 12, 103; and V. N. R. Pillai Synthesis 1980, 1-26. Various substituted analogues have been prepared in order to maximize the photochemical efficiency and chemical yield, and to supress colored products of the photolysis.
The o-nitrobenzyl group has been known as a photocleavable blocking group for some time (see, J. Barltrop et al. J. Chem. Soc. Chem. Commun. 1966, 822-823). The o-nitrobenzyl group has been used to protect many different functional groups, including carboxylic acids, amines, phenols, phosphates, and thioIs.
European Laid Open Patent Application No. EP 588,717 describes the use of o-nitrobenzyl blocked stabilizers for photothermographic articles. These compounds stabilize the silver halide and/or minimize leuco dye oxidation without causing desensitization or fogging during heat processing. Deblocking to release the parent stabilizer is by actinic radiation and does not occur during processing or during shelf aging.
Although classical methods for the conversion of carboxylic acids to their corresponding hydrocarbons (such as the Hunsdiecker reaction) are well known, none have been used to photorelease stabilizers in photothermographic elements. Okada et al. J. Amer. Chem. Soc. 1988, 110, 8736, reported N-acyloxyphthalimides as excellent deblocking groups for carboxylic acids. Decarboxylation occurred readily with visible light .lambda.&gt;350 nm. These authors also reported that decarboxylation can also be achieved using ultraviolet light in the presence of hindered bases such as 1,4-diazabicyclo[2.2.2]octane (DABCO), (see Okada et al. J. Chem. Soc. Chem. Commun. 1989, 1636). The authors used the N-acyloxyphthalimide group only for blocking carboxylic acids. Release of alcohols, amines, or sulfides such as those contained in photographic and photothermographic elements were not discussed.
Although phthalimidization techniques have found application in a small number of synthetic designs and technologies, phthalimide blocking groups have heretofore not been effectively employed in protecting the materials of photothermographic and dry-developable imaging. Thus, there has been a continued need for improved post-processing stabilizers that do not fog or desensitize the photographic materials, and stabilizer precursors that release the stabilizing moiety at the appropriate time and do not have any detrimental effects on the photosensitive material or user of the material.